The present invention relates generally to systems for producing a frequency modulated signal and more particularly to means for directly modulating an r.f. carrier to achieve substantially distortion-free FM within a given deviation bandwidth.
In almost all frequency modulated oscillators there exists the requirement that the carrier be set to a precise frequency and then deviated linearly over the peak deviation bandwidth. Among other desirable properties, such a linear response minimizes signal distortion. At present, the standard method by which an oscillator's frequency is set and then deviated is by means of a tank circuit having a voltage variable capacitor (varactor). The modulating voltage applied to the tank circuit causes a variation in the capacitive value of the varactor and hence a corresponding variation in the resonant frequency of the tank circuit. The resonant frequency of the circuit is related to its capacitance in a non-linear manner and the varactor voltage-capacitance characteristic is also known to be non-linear. It can be shown that the resultant center frequency response of the oscillator to an applied modulating voltage waveform is inherently non-linear for all deviation bandwidths. Thus, it is presently a difficult task to deviate a carrier linearly over an FM bandwidth with existing components and tank circuit designs without external compensation or special varactor diodes. External compensation often introduces additional and burdensome system complexity and cost.